In the production of lime from limestone or cement from starting ingredients, long cylindrical-shaped rotary kilns are often used. In the production of lime from limestone, for example, a charge of limestone is charged to one end of the kiln and passed therethrough, while a fuel is also charged along with air to support combustion, and the limestone calcined to form lime which is then discharged from the discharge end of the kiln. The fuel and combustion air are usually fed to the interior of the kiln adjacent the discharge end for the calcined product. Such fuels are generally gas or coal. The kiln is a long cylindrical vessel having an outer metallic shell with an interior surface of the shell lined with refractory material so as to withstand the high temperatures present within the interior of the kiln. As the material within the kiln is calcined, the high temperatures therein heat the steel shell through the refractory lining and heat is transferred through and radiates from the shell, which is lost, and is both environmentally and economically disadvantageous.
Heat losses through the shell of a rotary kiln represent a significant amount of energy and significantly increases the amount of energy input required for the production of calcined material in a rotary kiln.
Efforts have been made to prevent the loss of heat from such kilns and have generally focused on reducing the rate of heat transfer through the shell, primarily through the use of different refractory materials and shape of such materials inside the shell of the kiln. There are, however, limits to the effectiveness of internal refractory insulation, primarily related to temperature limitations of refractory materials and physical strengths of such refractory materials.
It is an object of the present invention to provide an apparatus and method for recovering heat lost from the shell of a rotary kiln used for calcining materials.